Heat treatment of wood is known in the art to improve dimensional stability and resistance against mould and decay. The effect of heat treatments on decay resistance of wood has also been studied by several researchers and it is well known that heat treatment increased the resistance of wood to decay, rot and fungi (see, for example, Kamdem et al., “Durability of heated treated wood” (2002) 60 Holz als Roh- und Werkstoff 1 and Hakkou, et al. “Investigations of the reasons for fungal durability of heat treated beach wood” (2006) 91 Polymer Degradation and Stability 393). Improvement of decay, rot and fungi resistance is especially notable when wood is treated at temperatures in excess of 180° C. The durability and decay resistance increases with treatment temperature and is reported to be complete when treatment is performed at 280° C. Several reports in the literature agree that chemical modification is the most plausible hypothesis to explain improvement in durability through thermal treatment.
The prior art discloses several processes for the thermal treatment of wood using different heating vectors including air, oil or liquid solutions. For example, U.S. Pat. No. 5,678,324 discloses a heat treatment process where the wet cellulosic product is dried in an initial step to 15% moisture content then subject to a temperature typically from 180° C. to 250° C. for 2 to 8 hours. Finish Patent No. 68,122 discloses a process for treating wood products at temperatures of 160° C. to 240° C. and pressures of 3 to 15 bar whereby the capacity of wood to absorb water is reduced considerably thereby improving dimensional stability.
German Patent No. DE 3043659A1 discloses a process which consists of immersing wood in an oil bath at temperatures of 130° C. to 140° C. This process is reputed to improve dimensional stability and durability. The improvement of durability is thought to be accomplished through a preliminary treatment with biocides.
U.S. Pat. No. 5,451,361 discloses a heat treatment process which produces high value products from low quality wood. This process consists of applying different stages of thermal treatment where the first stage is a softening stage where the wood is heated in the presence of an aqueous medium and at equilibrium pressure of operating temperature, to a temperature in the range of from about 120° C. to about 160° C. The second step consists heating the wood to a temperature in the range of from about 160° C. to about 240° C. for not more than 1 hour followed by dewatering and curing stages. In this process, the differential between core and surface temperature is used to pass from one stage to another. This process is time consuming as each step lasts for several hours.
The use of heating to reduce oil viscosity thereby facilitating wood treatment is discussed in German Patent No. DE4112643. Heating improves wood impregnation by the treating oils but only permeable species may be treated using this process.
U.S. Pat. No. 6,217,939 describes a wood treatment process using linseed or rape (canola) oil. The process described therein consists of treating lignocellulosic materials for several hours in liquid bath of oil with the exclusion of oxygen and is carried out at temperatures ranging from 180° C. to 260° C. for several hours. The improvement in the dimensional stability and resistance against wood-destroying organisms is reported as being due to the alteration of the lignocellulosic material by thermal action.
The available thermal processes using conventional heating vectors such as air, steam and oil are time consuming and need generally to be applied in an oxygen rare atmosphere under pressure, thereby requiring a closed reactor vessel. These processes are generally very expensive and energy consuming and as a result are typically not accessible for smaller businesses and wood producers such as pallet and furniture manufacturers.